I. TECHNICAL FIELD
The present invention relates generally to an electronically controlled automatic transmission capable of independently learning the fluid fill volumes of its friction elements and, more particularly, to a volume learn sequence which enables the transmission to learn these volumes before the vehicle has been driven.
II. DISCUSSION
In the present design of automatic transmissions, a series of clutches or friction elements provide a means for application and release of separate members to and from each other during the flow of power through the transmission. These clutches thereby constitute the means by which gears within the transmission are selectively engaged or disengaged from either the engine crankshaft or the transmission case. Four speed transmissions, of the type which this invention is directed, generally include any number of elements or clutches which are applied or engaged in various combinations in relation to each of the vehicle's gears. Those in the present transmission include an underdrive clutch (UD), an overdrive clutch (OD), a second gear clutch (2C), a fourth gear clutch (4C) and a low/reverse clutch (LR). In addition, the LR clutch element has an overrunning clutch device connected to it. Each of these clutches generally includes a plurality of alternating clutch plates and clutch disks which, when applied, engage one another and which, when the clutch is not applied, are free to move or rotate relative to each other.
To apply each of these clutches, an electronically controlled hydraulic fluid actuating device such as a solenoid-actuated valve is used. There is typically one valve for each clutch, with the exception of the overrunning clutch, an underdrive clutch (UD) solenoid-actuated valve, an overdrive clutch (OD) solenoid-actuated valve, a second gear clutch (2C) solenoid-actuated valve, a fourth gear clutch (4C) solenoid-actuated valve and a low/reverse (LR) solenoid-actuated valve. However, in addition to the OD solenoid-actuated valve, an MS solenoid valve also feeds the OD element when the driver selected operating condition is drive. The same MS solenoid valve also feeds the 2C element when the driver selected operating condition is manual 2nd or low. These valves each control fluid flow to a respective clutch apply cavity. The flow of fluid into a clutch apply cavity results in the application or engagement of that clutch. Fluid flow is enabled by the opening of the solenoid-actuated valve in response to command or control signals received by the solenoid from an electronic control system.
The electronic control system typically includes a microcomputer-based transmission control module capable of receiving input signals indicative of various vehicle operating conditions such as engine speed, torque converter turbine speed, transmission output shift speed (vehicle speed), throttle angle position, brake application, predetermined hydraulic pressures, a driver selected gear or operating condition (PRND2L), engine coolant temperature and/or the ambient air temperature. Based on the information contained in these signals, the controller generates command or control signals for causing the actuation of each of the solenoid-actuated valves which regulate the application and release of fluid pressure to and from the apply cavities of the clutches or frictional units of the transmission. Accordingly, the controller is programmed to execute predetermined shift schedules stored in a memory of the controller through appropriate command signals to the solenoid-actuated valves.
Although in the manufacture of such transmissions each of the transmission components is machined to precise predefined dimensions, manufacturing tolerances or build variations often result in components having slightly larger or smaller dimensions. This may ultimately affect the hydraulic fluid fill volumes of each of the various clutches, or in other words, the volume of fluid which must be displaced to effectively apply or engage that clutch. These fluid fill volumes are used by the electronic transmission controller to effectively control fluid application to each element in order to provide an optimum shift quality or feel.
To account for build variations in transmission assemblies, methods for determining the fluid fill volumes in each assembled transmission, either during the life of the vehicle or immediately after assembly, have been developed. One method for determining fill volume immediately after assembly is generally disclosed in U.S. Pat. No. 5,456,647, entitled "End of Line Volume Learn sequence of Friction Element Fill Volumes For Automatic Transmission," issued on Oct. 10, 1995 to Holbrook. This patent is owned by the Assignee of the present application and is herein incorporated by reference. In this reference, the transmission control module determines the fill volume of each friction element after the transmission has been assembled. To determine the fill volume, each friction element is independently actuated while only one other element remains engaged. Torque converter slip, speed difference between the transmission input and the engine, signals when the friction element is engaged. The transmission controller senses this slip and is programmed to update originally stored fill volume values and learn from the results of its operation. A nominal volume value is stored in a non-volatile memory location but a learned volume value is stored for use by the transmission control logic in a battery backed RAM. This learned value is updated as each friction element is actuated and is thereafter used by the transmission controller to precisely control fluid flow and fully optimize shift quality. However, the application of this method to the present transmission has several drawbacks.
First, the transmission which the prior art is directed to contains four friction elements (an OD clutch, a UD clutch, a two/four shift clutch, and a LR clutch). The presence of the 2C and 4C clutches in the present transmission replaces the two/four shift clutch which the Holbrook method is directed to, thereby adding another friction element unaccounted for in the Holbrook device.
Second, The transmission which the present invention is directed to contains an overrunning clutch (ORC). The addition of the ORC prevents inputing engine torque through application of the UD clutch, as is done in the '647 patent while learning the volumes of the other elements. When the fill volume of the LR clutch element is being learned, and thus being filled, it is also necessary to maintain some fill in the OD element. The fluid pressure in the OD element must be maintained at a predetermined level low enough to allow the LR element to be filled and high enough to transfer some torque to the gear set.
Third, the present transmission provides dual feed to the OD element through the MS and OD solenoid valves while the shift lever is in the drive position. As a result, the OD clutch element fills faster, by virtue of there being two feed paths, than it would if only one solenoid was filling the element. Since the normal operation of this transmission uses both feed paths to apply the OD clutch element while operating in drive mode, it is necessary for both to be used when determining the fill volume. The transmission which the Holbrook method is directed to does not contemplate such a use. The present invention was developed in light of these drawbacks.